Originally Posted by Oneirogenic
Agree and disagree with this. Lighter gauges and lower tensions are nice for uptuning but our OP may prefer a wound string for his highest string. I don't mind plain strings at all but some people do. The feel and timbre difference can be very weird especially if you haven't played instruments that have plain strings. OP may also want to consider that very light gauge wound strings are far more prone to breakage because of the very thin cores they have.
.035 doesn't seem particularly loose for a "Bb". Tight, if anything. I once tried a .035 as a "C" string gauge (a step higher) and it broke while I was stringing it up. 35" scale though, to be fair.
For the "Eb" string, Conklin sells a 0.20 wound "F" string which should work fine. Guitar strings are also an option if they fit (I used to use a D'Addario guitar string (wound 0.28 "D" on my 35" scale bass - just barely fit).