Yes you can make beams with just a mini mill that are straight and square.
Yes you can use regular cross cut chain for milling with fine results.
Straight is based on how well you chose your guide board and attach the rail sections to it.
Square is based on how well you set up your guide board to the log, the first cut is the most critical and should take the longest to set up. If the first cut has a twist it will carry out to the next three cuts.
For the first cut I would recommend cutting square notches in the log (cross cut way) and attaching small chunks of 2x4 to the log to support the guide board. Space the notches 3' or so appart, use a string and a level to set them all up. Set the ones at the ends of the log up first (make sure they are square and level to the intended cut) and run a string between the two, set all the other support 2x4s even with the string and make sure they are level. I usually use ledger locks to attach the guide board to the log, 4" ones seem to be fine. Becareful in oak though because sometimes they break off after multiple uses, so I use soap (bar soap) to lube the screw.
Best of luck
that's blue spruce. I was getting lazy let the chain get too loose, too dull and was forcing the saw thru faster than I should have. It was the end of the day and I wanted to finish. 
