Congrats. I just finished my fixie too about a week ago. Its a blast.

For chain tension, I just pull back as hard as I can on the wheel and have someone wrench down on the drive side bolt. I'm looking into getting a chaintensioner sometime. (maybe a mks allen bolt tensioner if I can find them)

You could try getting a shorter bottombracket so the outer position on the spider lines up with the cog. I run an old Shimano 105 double crank and I use a 107mm length BB to get the outer to line up with the cog. I wouldn't use spacers on the cog