Would it be ok to keep two red tailed black sharks in a 15 litre hex tank?
Would it be ok to breed two guppies in a 15 litre hex tank?

No, the sharks should be in a 70 gallon i read (im not sure if this is true or not..)
15 litters is only 3 gallons, i would put the guppies in at least a
5 to 10 gallon when they breed they never stop ... and you could get
up to 50 fry a batch,thats alot of fish.

red tailed black sharks, if they are the ones I think you are referring too, should really have 30 gallons by themselves once full size. Sooner or later they are going to get about 5-6 inches and are aggressive towards each other once full grown. If you have 2 that are fairly small, they can be kept together as long as you have hiding spots for them and once they get bigger, either seperate them or get a bigger tank with more hiding spots.