There is no website that lists out every possible breeding matchup and says whether it is compatible (or what their breeding rate is). Such a thing is highly unnecessary, as breeding compatibility and breeding rates can both be distilled down to a simple table, like this one on Bulbapedia. This table explains everything you need to know, but I will also give an explanation in this answer.
I think some terminology is being muddled in this thread, so to be absolutely clear:
- Breeding compatibility refers to whether a pair of Pokemon will ever breed; either "yes" or "no". Breeding compatibility is no more complex than this list of rules:
- If one Pokemon is male and the other is female, and they share any Egg Group except Undiscovered, then the Pokemon are compatible.
- If one Pokemon is Ditto and the other is not in the Undiscovered Egg Group, then the Pokemon are compatible.
- All other pairs of Pokemon are incompatible.
- Breeding rate refers to the chance that a compatible pair will produce an Egg. This chance is applied each time the game attempts to generate an Egg, which it does with every 256 steps you take in-game (so you can try an infinite number of times by walking/biking forever). The only two factors affecting breeding rate are a) the species of the two Pokemon and b) the OT of the two Pokemon.
The table on Bulbapedia is showing the breeding rates. The breeding rate is highest when the species is the same and the OT is different. If the Pokemon are not compatible, then we say the breeding rate is zero.
To emphasise: if the Pokemon are compatible, then the breeding rate is always above zero, so all you need to do is walk/bike around a lot in game and the Pokemon will eventually breed. People tend not to care about the breeding rate for compatible Pokemon, because that's always at least 20%, which is quite high.
To connect to your scenario: your Kangaskhan and Slowpoke are compatible as long as they are opposite genders. You cannot change their breeding rate because you cannot make Slowpoke and Kangaskhan have the same species and you cannot change a Pokemon's OT. Their breeding rate depends on whether their OT is the same; use the table on Bulbapedia to see the difference this makes. As explained above, the level of the Pokemon is not a factor for the breeding rate, so levelling up Slowpoke will not affect the breeding rate.
(In Black/White 2 onward, you can boost the breeding rate with an Oval Charm. In Gen 2, breeding rates are considerably lower than in other generations, and there is an extra way for Pokemon to be incompatible that depends on their IVs. Again, all of this is covered comprehensively by the tables on Bulbapedia, to where it is very pointless to list out every possible combination.)
