What if I only put a little amount of head gasket sealant in my radiator?

It will not matter to much, but you must follow the instructions that are on the product when using any sealant as the cataylsit for the sealing action may or may not require heat to do so. You may end up pouring it in and cloggin psrts like the thermostat or other issues may occur if you do not follow them. The sealant is usually a mistue that does not stay mixed to gether well and requires the material to be mixed by the action of the cooling sytem and the heat starts the catylist porcoess which then the product hardens when it comes into contact with the air. You probably did not put in the elements that would actually create the mixture for sealant to work. I cannot say that it will difniately not cause a problem because I do not know if you used a quarter of a pint or a half of a quart and the type of sealant would make a difference to as some off the cheaper ones are not made of several fluids/materials and just need the heat to seal small holes, so in either case you are more than likely okay. However deopending on how much you out in, the sealant may no longer be any good as if it is a multipart type of sealant you poured of the lighter or les dense liquid and the ratio will not be correct now as you do need to frain some coolant out to make room for the sealant. Expensive sealants require that you ablsolutely make enoguh room and then get some 50/50 premix antifreeze to top off. They al;so require the engine to be hot and/or cold at certain points and not to be running, etc.
So, you are probably oaky and just read the instructions before trying or making any attempt to use again.