I have guineas and have found them to have good fertility in the temperatures that you mentioned. I would recomend against cross-breeding guineas. There are already many experienced guinea breeders creating new color varieties (the last count I saw was 34 different varieties) and I think that it is better to work on improving the varieties that already exist. In my own experience some mutts are nice, some are ugly, but most are so close to one or the other of the parent stock that most people would confuse them with purebreds. For this reason it is best to stay with purebred breeding. Breeding pens are always a good idea so that you can insure a good fertility rate with the proper male to female ratio and only breed the better birds (not the smaller or poorly looking ones). The only cross breedingI would reccomend is creating pied guineafowl. Many people find these to be very good looking. They can be obtained by crossing any white guinea with any colored guinea (although i recommend crossing the white with something dark such as chocolate, purple etc) not something light like buff duddotte or porcelein as it will be hard to see the pied effect. Be careful if you try to get pieds make sure that you have a white guinea, some of the light varieties can look like white guineas but wont produce a pied effect. If you think you have a white guinea look for spots on it. If it has any spots then it is not a white but some other light colored variety. Whites do not have any spots.
Hey thanks a lot you guys. Yeah those pied guineas are awful pretty. I will have to set up a breeding pen and try to match out the proper eggs. Friend says he knows he has males but isn't always 100% sure which they are. Going to be a fun week learning how to sex them and then cornering them and banding their legs appropriately.
As for sexing guineas I think the best way for most people is to listen to their calls. the males only have a one note alarm call and the females have a two note call that is supposed to sound like "buckwheat". I think it takes a bit of creative interpretation to think the females sound like they are saying buckwheat so just remember if it makes a two note call it is a female. The females helmets are also are lower than the males and tend to be sloped back more, however there can be a lot of variation in helmet size and one of my females has a helmet bigger than that of most of my males. The males helmets tend to be taller and the point more to the center. Also dont worry too much about having multiple males in a breeding cage. As long as they are not really closely confined they normally get along fine. If you are just starting out breeding guineas you could probably just throw all the ones of the same color that you want to breed in one pen and call it good (assuming you think you have a male and a female in the group). I have all my guineas in a common pen when I am not breeding them to make it easier to free range them and I have had little trouble with males fighting. I had one very aggressive one, but dinner solved that problem.