It sounds like you've been playing autoconvert maps (maps made for standard, not mania). Autoconverts have different numbers of keys depending on the difficulty or something (i actually forget exactly what determines how many keys).
I'd suggest downloading some easy mania specific maps instead of playing autoconverts. When you play a standard map in mania, it has to try to convert the standard map into a mania map, and that causes a lot of problems with how it decides what patterns to use. There are some patterns that appear in autoconverts that you will almost never see in a proper mania map, and there are a LOT of mania specific patterns that you will never see in an autoconvert.
Mania maps require at least 2 difficulties for each key mode that they contain, but can contain multiple key modes. The most common key modes in mania are 4k and 7k, although there are some ranked maps for the other key modes too. This means you can stick to one key mode if you want to.
The scroll speed issue you mentioned is because by default, osu multiplies your selected scroll speed (the number in the top right corner of the screen) by the bpm of the song, so if you find an easy map that is 200 bpm, it will be twice as fast as an easy song with 100 bpm. gintoki posted a screenshot showing how to disable this. If you disable bpm based scroll speeds, you will probably have to increase your scroll speed a bit in order to find a speed that feels right. To change scroll speeds, use f3/f4.