So, i am trying to make music using Litterbox mml, but i can't seem to figure out how to make my music longer than 4-5 seconds. Does anyone know how to stretch out the length?

Click on the top left (pattern arrangement). Try messing with these, I'm not sure which one helped but I got it to 10 seconds, not sure of the memory limitations but, with this many patterns at 10 seconds each, you should be able to play them all sequentially somehow. That would put you over 1 minute.

Mind you I've hardly used Litterbox, but I think thats the way to do it. More measures usually means longer I think.

I've been able to port a lot of song from synthesia or by ear to smilebasic using litterbox. To make song that are longer, you need to create a new page(With the button new). Then, to hear the whole song you need to press L+X on the first page. By the way, the pic attached display the number of page that I used to port a song called Coalescence from Risk of Rain.

Do either of you know if it is possible to use multiple types of instruments in a song?

Yes, each channel can use a different instrument for the whole page. If you need to change an instrument of a channel, then you must create another page and set other instrument on that channel. Also, you can quickly change an instrument by clicking in a channel while pressing L or R.