IMHO, you do want to compress the suspension. I don't like the CG of the trailer shifting as I'm driving. I know that others feel that you should not compress the suspension but I'm old school IRT towing. Whatever tie-down approach you use, adding wheel straps is good insurance.

Sorry but haven't a clue how to estimate max capacity of a trailer with no specs available. I'm sure that someone in your area (UHaul maybe) can give you an estimate based on spring size, hub size, receiver class, & other factors.