Front wheels see a lot less loading than rear wheels, so if you're building the rear with 36 I wouldn't worry about 28 on the front. Actually, if you build both wheels with the same # of spokes (and gauge) and same rims, either the front is overbuilt or the rear is underbuilt.

Finding matching rims with different hole counts can be difficult though. And personally I like my wheels to match if I am going through the trouble of building them up by hand. See what you can find for rims, otherwise I'd suggest selling the 28h hub and getting a 32 or 36h if your rim choices are that limited.

I'd take that 28h Phil hub off your hands if you really don't want it