Have no idea if you can transfer trial account characters, but I am sure that they are limited to level 20 as the max level so you will need to upgrade all 3 trial accounts to full classic accounts if you plan to instance boost them using RaF.
If your main account invites all 3 other trial accounts it will be linked with them and as such the lowbies will only recieve the normal instance xp, as the veteran account character is greater than 3 levels higher than them.
The way you would need to do it is get a friend to send you a trial invite that you will then need to upgrade to classic WoW. Once the account is upgraded use that to send yourself another 2 invites via RaF and upgrade those to classic WoW accounts also. Now those 3 RaF accounts are linked you can boost them through instances with your main account and all 3 lowbie characters will recieve instance xp plus the RaF bonus.
Thx a lot for the fast reply,
And i forgot about the lvl 20 max on trial accounts, so thx for pointing that out to me. But now I fully understand what I need to do, to get the max out of RaF!
I'm going to try this out tomorrow when I get back from work, if I find anything that could be useful to other starters, I'll post it here too.