From the side of the RadMap control I don’t see any problems to show control with fixed size inside the information layer. I also think that this is not a problem to create data for RadHeatMap. But there are few limitations on the RadHeatMap control. This is what I've got from my colleague who is working on the RadHeatMap control:
It is not possible to set a fixed size for the cell of the HeatMap. The HeatMap checks how many rows and columns are there and how much is its size and based on these it calculates the cell’s width and height. However, if you set the size of the HeatMap to be 100x100 and you have 10 rows and 10 columns this will surely result in each cell having 10x10 size (provided we are not showing any labels).
Currently we have not implemented a CellClicked event, but I am sure that if you know how many rows and columns you have loaded, you can determine over which cell is the mouse when the MouseButtonDown event is raised.
the Telerik team