well, my first comment is that summary of the ratings differences is utterly impossible to follow - as is expected. i don't have time to really get into it, but you should be looking at the player's abilities, not just ratings, which has the positive side effect of making it much easier to talk about this stuff. iwanturmind had a question recently where i got into the difference in abilities vs ratings to some extent, maybe that would be a useful read for some background.
anyway, definitely agree with parker at pg and walthall at sg. urick cant play pg so the question is really which of the other 2 plays pg and then who plays sg, it seems to me, as a way of simplifying a bit. devita and johnson are roughly on par defensively (the ability, not the rating) and in terms of guard skills, particularly passing (passing matters extra at pg). offensively, its a bit of a wash as well - devita has better core 3pt/fg% ratings, but johnson has IQ and ft%. i think you could go either way, but by the end of the season, probably devita is the way to go.
as far as the sg situation, it depends what you need. it seems to me you already have enough 3 point shooting and perimeter offense from the 2 starters and devita at backup pg (or johnson, if you so choose). urick is stronger defensively than the other two, and from a guard skills standpoint, they are roughly even (bh/pass). his offense is probably a bit less than the others, so really its a trade off - not a strict "one is better than the other" type of thing - as is often the case. it all comes down to what abilities your team needs and what abilities these players provide. it seems to me, without looking at the rest of your team, that you have plenty of guard offense already, so i'd guess urick. but if you have crap offense outside your guards, you might want 2 offensive guards on the floor at all times. and maybe urick would work out fine scoring wise, that seems to vary by situation for a player like him, so there's that caveat, too.