Are all the blocks the same st patt? (You mention they diff in size from 9" to 10". Just yarn weight diff?) If the same, do a test on one and see how many sc are needed to make a clean, flat edge. However, if the blocks are all diff, your sc count may vary. You can still join them even if the st counts aren't the same. You may have to pick up every st on one but skip a few on the adjacent sq.
The afghan in the attached pic has no two sqs alike...all diff st patts...but all had chain sel and sc border (different no of sc on each). But they seamed together without any probs.
Keeping to the knitting basics