A big consideration with glass fusing is glass compatibility, one factor of which is COE. Most things in our world expand and contract in response to temperature. With glass, if two pieces expand and contract at different rates, they will crack.
A lot of the glass people use for fusing is “tested compatible”. Different bottles may or may not be compatible with each other. A single bottle is only compatible with itself. So… you may have varying rates of success at trying to fuse together different bottles.
All that being said, check out the Glass with a Past web site for a lot of good info on fusing with recycled glass. Jodi McRaney Russo has a lot of tutorials: http://glasswithapast.com/
Fused Glass Designs
dba Jester’s Baubles