Taking the time out to learn about the animals you keep is never a bad thing! Glad you found our fishy forum!
Very good to hear that your gourami are getting on well together, that gives you a little bit of time to find the Blue a new home before he gets too big for the tank - but I wouldn't wait too long. Generally, fish that are brought home from the shop are young and have a lot of growing left to do. A fish the size of a Blue Gourami will become stunted in a 10g tank, which will not only affect their overall size, but the rest of their internal growth as well. This leads to a very unhealthy fish with an unfortunately short lifespan - not a very kind thing to do to a creature.
Eventual size isn't the only reason the Blue needs a new home, though. These fish have probably not reached breeding age at this point, which is likely why they seem to be getting along. Gourami can be quite aggressive when their thoughts turn to romance. Regardless of there being a mate in the tank with them, they will pick out their ideal territory and begin to make bubble nests in the hope that the girl of their dreams will come along soon. Neither of these fish is likely to be very polite when the other gets into their way, and in a tank so small, chances are very high that they'll be competing for the same spot.
A heater and thermometer should definitely be on the tippy top of your to do list. These fish will do well at a temperature in the mid to upper 70's, and while I know people who are able to keep them without heaters, I wouldn't recommend it - especially not to a beginner. Ten gallons isn't much water, so the temperatures in the tank are able to shift fairly quickly depending on ambient room temps, and at this time of year (here, anyway) the temperatures can be dramatically different between day and night, which can cause stress on the fish. I'd recommend that you get a heater that will allow you to adjust the temperature manually, and not a 'preset' heater, as in my experience those tend to be a bit. . . tricky.
For the water tests, most of us on this forum recommend the
manufactured by API. Do avoid the test strips, as they can be highly inaccurate. The Master kit includes a Ph test, but not a test for Gh or Kh. You may be able to call your local water supply company and get the information on the water hardness in your area. When time comes to choose your next fish, you'll want to be certain to pick fish that will thrive in the water available to you - not all water, nor all fish, are created equal. More information on this can be found in http://www.tropicalfishkeeping.com/freshwater-articles/water-hardness-ph-freshwater-aquarium-73276/, if you'd like to explore this further.
One more thing that you might want to look a bit farther into is how to care properly for the live plants in your tank. They'll require nutrients and special lighting in order to thrive. I could get further into this, but you might be best served to put up a thread in our plant section, the gurus there will get you pointed in the right direction!
Because you mentioned the newness of your tank, I'd like to take a moment to direct you to our article on the Nitrogen Cycle
. Very important information to understand when keeping a tank. If this tank wasn't set up with established media from another tank, it could very well still be going through the cycling process - which is dangerous to the creatures who live there. Very likely that this could have had some impact on the death of the Plec. Or not! I don't know where your tank stands, but I felt I should point it out :)