Actually, Triangle palms are quite to grow indoors. The winter temperatures you describe are fine--the relative coolness will help avoid insect issues (scale and mealy bugs). You can add a light to highlight it if you wish but maintain some distance as you don't want to dry it out too much. Sounds like that room is quite bright, so it should be fine. I know a bright an unobstructed, large window in can offer terrific, indirect light--direct sun is not necessary to maintain the palm and it may even grow if the indirect is bright enough. After all, how much and how fast do you want a nine foot palm to grow inside your house anyway, If you do have it acclimated in bright, indirect light, be careful if you move it subsequently into bright sun (for example outside--if a terrace is available--because it WILL BURN unless gradually acclimated to direct sunlight. Triangles present no specially issue in transplanting (at least in my experience). But remember, palms much prefer to be slightly tight in there containers so I would refrain from transplanting (especially in the winter). Water thoroughly when dry (guessing this may mean watering anywhere from once a week to once in two weeks)--they are moderately drought tolerant. I would also wait until about March to start feeding and I would err in the direction of underfeeding rather than overfeeding. Good luck!

Your very lucky! These are beautiful palms! They should do fine where you have them. Dont keep the soil too moist and if possible, mist the fronds every once and a while just to keep humidity up a little bit and dust off the fronds. Dont transplant this time of the year, but if its in a very very small pot for its size, then you should pot it up into a larger pot (not too much larger) in the spring. Take it outside for the summer in shade when it first comes out, and then full sun once its been outside for a month, and it will be very happy with the hot summer days and water just about every day (but they can handle going a little dry, and should be a little dry in cool weather).
Good luck!
-Alex