You are mostly correct.
Although I'm sure this is not what osiris meant, I will clarify. This point actually addresses both of the statements you made above:
You are correct in saying the string determines the type of quark(yes there are 6 quarks, "up and down, charm and strange, top and bottom" 12 if you count their anti-matter pairs). However ALL strings are the SAME and EVERYTHING from matter to light to even force is made of strings (according to string theory). So matter is "created" (using the word loosely) by a string vibrating. There are vibrational patterns that result in 0 mass such as the theoretical graviton, and possibly the neutrino. The only difference between, say matter and force is simply the pattern in which the string vibrates.
I would also just like to add that of the particles you mentioned above only the proton and neutron are made of quarks. The rest are elementary. UP quarks have a charge of 2/3 while DOWN quarks have a charge of -1/3. a proton is composed of 2up and 1down quark so:
2/3 + 2/3 - 1/3 = 1
and a neutron is the opposite, 1up and 2down
-1/3 - 1/3 + 2/3 = 0
note: the fractional charges of the quark are based on the strength of the elctron charge, which relatively is one, which is why the protons charge cancels the elctron.
NOTE: the other quarks are the same as first two. up charm top quarks have charges 2/3 while down strange bottom quarks have charge -1/3. The only difference in the families is they get progressively more massive.
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