Can a regular heptagon whose angle measures total 900 degrees be used by itself to make a tessellation?

No, it cannot. I was doing my own tessellation project when I came across this question. In order for a polygon to tessellate, the sum of the angles surrounding one point must be 360 degrees (think of it this way, if you spin in a circle, you spin 360 degrees.) So, if a heptagon has a total angle measure of 900, each andgle is approx. 128.57 degrees. 128.57 cannot fit evenly into 360. Hence, a heptagon whose total angle measure is 900 cannot be used by itself to tessellate.