Music notation question

Interesting question really.. Had to read that twice to get what you were asking ;)
I think you have two options. Like you said, inserting a measure in another time key or trying to fit it in with the rest. You can just write them down in the next measure, and then move everything that comes next.
Or, that seemed logical when I thought of it. But now that I'm typing it, I realise that if you move everything like that, the accents might change. I would just fit in a measure in another timekey, that way it is obvious how you want those notes to be played, and the rest of the arpeggio will remain unchanged. But yeah, comes down to the point of this part of my post being useless as I can't give an "official" answer =P

I think you can make a 12/8 of it. It's just not being used that often, or well, I haven't encountered it that much yet. The only thing I don't know, is how that will change accents. You know, in a 6/8 the accents are normally on the first and the fourth 8th note. But, that all comes down to how you play it, right? So yea, another I don't know for sure here, just my vision of it ^^

There may be better answers:

1. You can change the time signature in the middle of the piece, but your choice depends upon which downbeats you wish to have emphasized (accented). This leads in the the second answer:

2. 6/8 is generally counted (tapped) as 2 beats per measure, one beat on the first of each set of eighth note triplets. The first downbeat (first note of the first eighth note triplet) typically receives slightly more emphasis, than the second (first eighth note of the second triplet). If the time signature is 12/8, one will count it out as four beats per measure, once again giving slightly more emphasis to the first downbeat in the measure. So the feel of 6/8 is one slightly stronger accented and one less strongly accented downbeat per measure of 6 eighth notes and 12/8 is one more strongly accented and three less stongly accented downbeats per measure of 12 eighth notes. Sometimes a subtle difference.

I would add the 2/8 measure, and not a measure of 4/4, simply because of the feel of a 6/8 measure would be different in 4/4. This isn't an uncommon thing to occur in a song, i can remember playing stuff on drums that changed time signatures frequently. I personally think that the difference between a 6/8 measure and a 12/8 measure is the idea of what accents fall on what beats, I've found that in some songs in a certain key, the root of that key will fall on beat one so it makes sense why it's in a certain time signature. Anyways, adding the measure of 2/8 will allow you to keep thinking: one beat = one 8th note, where as in 4/4, it might be a bit confusing.

The difference between 6/8 and 12/8 is just the feel of it. In every measure you want to have a thought, you should be able to draw in the lines of your measures just by looking at the notes, and not counting any beats at all. If it feels like you have one idea repeated exactly twice after 12 beats, use 6/8. If it feels like it takes 12 beats for every idea then it's 12/8 time.

This relates to your other question. Are the added notes the end of the previous measure, or the beginning of the new measure? It's up to you which measure to include them in. I personally would probably include them in the previous measure, making that a measure of 8/8.

I think there is some Russian song that alternates between 5/4 and 6/4 time, every measure, or something to that effect.

The way you notate measures really depends on the feel of the song, and how much you are saying per measure.

Don't worry too much about it all, it will all sound the same no matter how you notate it :)

The accents are the key to the whole thing.

6/8 time is a duple-compound meter, meaning you have two accents - a primary accent on note one, and a secondary accent on eighth note four; the two stresses each divide into three parts.

When you have your two extra beats, the first one is probably stressed. That gives you one measure of a duple-simple meter. So you could show it as a measure of 2/8.

If you make one measure 4/4 to account for the extra beats, you'll be shifting the accents - 4/4 is a quadruple-simple meter - the stresses go on beats one and three. That means your second beat in the 6/8 meter before the extra notes will lose its stress, as will the first of your extra two notes, and it will instead fall on the fifth eighth note in the measure... the beginning of beat 3.

4/8 has the same drawback.

Now, if your first note in the 2/8 measure isn't stressed as strongly as the other 'one' counts, you need a different solution - to keep the stresses in the right places, you could take the measure before your extra beats and split it into two parts. The first part would be 3/8 (a triple-simple meter, keeping the stress on your first beat); the second part would be 5/8.

5/8 is an odd time signature, meaning the secondary accent can be interpreted as the third beat (combining one duple and one triple pattern) or the fourth beat (one triple and one duple pattern). You'd need to use an accent mark to place the second stress over the fourth eighth note for clarity.

This would turn the second stress in your original 6/8 measure into a primary stress, though... so figuring out which notes are really emphasized will tell you what the best solution is.

For the second question, the difference between 6/8 and 12/8 is pretty subtle. If all your stresses are the same, it's 6/8. If every other one is a little stronger, it's 12/8 - the first eighth note is a primary stress, and the seventh eighth note is a secondary stress.

If you ever played in a marching band, it was a lot less subtle!

Thanks folks. The beat falls on the 1st and in a lesser way on the 4th. Every first beat is the same. so I guess that makes it 6/8. The 2/8 measure has the emphasis on the first note, so that one stays 2/8. As for the single 4/4 measure, that one has the emphasis on the one and three, so I can keep it like that. Think I got it now, thanks a bunch!

LOL yeah, I did marching band in high school (on snare drum). And if you hung out in some of the rowdier blues clubs, there ain't much difference between stresses' in a rollicking 12/8 tune either :)