Planted Tank Obsessed
Join Date: May 2011
Location: Edmonton AB Canada
Might be a bit small for 2 guppies... they need 2 gallons minimum, which the spec is yes, but I'm not sure how much of the 2G is made up of the filter area in the back. You should be able to shove your heater into the space where the pump and flexible hose sits so you don't have to put it in your main tank area, I've done this on a couple specs and it works well and still keeps temperature even t/o the aquarium.
Some people here won't be too happy about you using live fish to cycle on the basis it is cruel and can be torturous to the fish (which is true I guess, can damage their gills for the long run) so another good way to do it is to just get your tank up and running and every other day add a pinch of flake food and let it sit in there and rot thus providing an ammonia source. However if you can get some filter media from someone with an established tank you can just shove it in beside the bio rings in the spec filter and with adding some flake food you should be able to be fully cycled within a week, depending on how much media you're going to be able to add.
Good luck and have fun, don't get discouraged when things go wrong at some point in time which I assure you they will. It's normal