If you're looking into heaters, you'll want to find something thats adjustable with a thermostat and fully submersible, about 25 watts should do. Stay clear of those 'present' heaters, though they're cheaper, they usually under-heat(though I've heard of a few cases where they've over-heated)or don't even work at all. Hydor, Elite, Aqueon, and the Petco brand are all good heater brands.
Also, I ordered the heaters in my smaller tanks(2-4 gallons)for about $6 on ebay and they worked out very well. Only problem is shipping takes a couple of weeks, and the cord is rather short so you'd need a small extension cord probably.
http://www.ebay.com/itm/-/271031514998?item=271031514998&ViewItem=&ssPageName=ADME:L:OU:US:3160
A single female might work out a little better....sorry I forgot to mention that, I was even thinking about it while responding, lol. Females do tend to be a little more docile, so one would probably tolerate the guppies a little better then a male. However......females CAN turn out to be just as aggressive as any male. But you have a better chance with a female getting along with the guppies.
Breeding bettas isn't something that should be taken lightly....its a VERY expensive and delicate process, its not like guppies where you toss in a male and female and they just breed. Bettas are aggressive fish, selectively bred for many years to FIGHT each other....there are many cases of, even in the best conditions, males and females killing each other during breeding, and rarely do they escape the breeding tank without at least a nip or two.
You need to provide the right conditions, you need to know when courting crosses the line into real aggression, you need to have a goal and a plan with your breeding(colors, genetics, form...), you need to be able to provide a variety of live food for your fry as well as be there and have the time to preform daily water changes and feed 3+ times a day, you need to have enough containers for any males or aggressive females you might end up with out of the possible 200+ offspring and the time to change THOSE daily when they get to the point where they need to be separated, plus you need all the funds for setups(at least 10 gallons for spawning, and 20+ for a grow-out, plus separate containers)and equipment and live food and whatnot.....
Basically, breeding is kind of a huge endeavor.
If you are really interested in breeding.....don't rush into it. Take some time, at least a few months, and research research research until your head hurts then research some more. Slowly collected what you need, hunt around garage sales and thrift stores and Craigslist for tanks and jars/containers, and make a plan. find out what sort of colors/patterns/tail types you want to work with, get a nice pair off Aquabid.com or from a local breeder if you can find one(its usually not recommended to breed Pet store bettas since you have no idea about their genetic background unless you have an LPS or two that would be interested in taking the babies or you have another good plan for possibly 200+ fry).
Not trying to scare you off of it, but you really don't want to take it lightly. Its a big deal, VERY time and money consuming, but it can be very rewarding and worth it IMO IF you are prepared and do it right.
). The issues with bettas and guppies, mostly in the case of male fancy guppies I believe, is that the aggressive betta sees another color fish with long fins, thinks its another betta, and attacks or that the guppies get nippy with the betta and bite at his long flowy fins. 
