I don't know much, but the ケ is definitely 'ke', and the ッ if it were larger is a 'tsu'. When you have a regular character followed by a small one, you sort of smoosh the sounds together. So in this case ケッ is probably 'ketu' or 'ket' (e.g. pocket).
Look at your chart at gya (gi and ya: gya), chu (chi and ku: chu) and you'll see how these work. To extend sounds out, you use a character that looks like an emdash (). It's used in beer (ビール bi-i-ru), for example.
I'm sure people better versed can go into more detail on the whats and whys...
Edit: If you're looking for a complete list of katakana, that chart is missing quite a bit. Not sure what they're actually called, but it's missing what I call 'modifiers'. e.g it has ハ (ha) but not パ (pa), has ス(su) but not ズ (zu). Note that from what I can tell the modifier sounds are usually made with the same part of the mouth.
