compact beef breeds to cross with our Angus heifers?

I would assume it's a possibility...
Black baldies often have horns from the Hereford side...

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~*~Erin~*~
SAHM, ranch wife, sub and quilt shop proprietress
the Back Gate Country Quilt Shop

The poled trait is dominant. So if you have a homozygous cow without horns, and put it with a horned bull, all the offspring will not have horns. But if the cow is heterozygous (ie one of her parents had horns and one didn't) then about half her calves will have horns and half wont.

sire - h h sire horned (hh) x
-----|----- cow homozygous polled (PP)
C P Ph | Ph => 100% heterozygous polled offspring
o -----|----
w P Ph | Ph

sire - h h sire horened (hh) x
-----|----- cow heterozygous polled (Ph)
C P Ph | Ph => 50% heterozygous polled offspring
o -----|---- => 50% homozygous horned
w h hh | hh

Often "true" BB's are polled. Any offspring resulting from a cross between a horned breed (Hereford or Simmental or Charolais or Limousin) and a polled breed (Red Poll, Angus) will result in a polled calf. By "true" black baldies I mean a calf that is an F1 (expressing hybrid vigour) cross of an Angus sire and a Hereford dam or vice versa.

However, in your case Erin if you're getting black baldies with horns chances are that the cross could not be from black Angus, but from perhaps a black Simmental crossed with a Hereford to get the black baldy that you described. Like Army Doc said, if the black Simmental sire is hetero for horns (physically polled but genetically has the horned gene [phenotypically polled but genotypically has both polled and horned gene]) and the Hereford dam is phenotypically horned (genotypically as well), then there's a 75% chance that the offspring will have horns, and a 25% chance that the offspring will be polled, but has a 100% chance of carrying the horned gene either way.

Edit: the black sire/dam could be a cross between an Angus and another breed which expresses the horned gene, resulting in having the sire/dam being heterozygous for the horned gene, which in turn gives you horned black baldies.

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Scottish Highlands cross nicely with Angus........

And offspring are polled.

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Do not meddle in the affairs of dragons...for you are crunchy and good with ketchup!

I would go with Lowline-- I have a very nice steer out in my pasture that we will be butchering this year-- he just turned one-- he is a Chunk-- has only been on pasture and local hay all winter.
Especially with your girls being heifers, you will want a smaller calf that is up and ready to go right after calving- and will put on the pounds with minimal input.

just my opinion,
susie