The official Gil Hodges website (gilhodges.com) implies that Hodges entered the majors as a third baseman. Why was Jackie Robinson at first at first (on first, first? first at first?), instead of the outfield or at his eventual position of second base? Why was Hodges, a third baseman, moved to first? I have a few ideas about first basemen that have been rattling around in my head for a while now ....
It was a different game back then. There was much less emphasis on the home run, much more emphasis on line drives, singles, grounders moving runners up, sacrifices, etc. Thus, there were not as many fly balls for speedy outfielders to run down in the deep gaps of the ballparks. However, more ground balls required better infielders at all four infield positions. Thus, Jackie Robinson was a great athlete playing first base, with the athletic skills to move to second base. When a comparable athlete was available, say, a third baseman with good reach and range like Gil Hodges, Robinson could make the move to second.
Why was Robinson not in the outfield? Think about Little League baseball. Where do the lesser-skilled players at the lower levels get stuck? Right field? Left field? Yep. At that level, with fewer fly balls leaving the infield, the need for good athletes in the infield and a good first baseman is much greater. (Well, most Little League coaches, anyway. See contrarian example, below.)
I think a lot of managers and coaches consider first base a 'throw-away' position in today's game. Put in a big guy with limited range (he only really has to move to his right, anyway), who doesn't have the speed or the arm to play the outfield, and as long as he can clobber the ball at the plate he can be an All-Star. I saw it when my lefty, natural first baseman son was a 10-year old. The coach put a fat kid at first, and put 'Cisco' behind the plate ... as a lefty! Cisco had a great year, but the fat kid never caught more than two throws from third in a row, and he had to hit the ball to the fence to get a single. Scoops for throws in the dirt to nail a runner, or moving even a couple of feet to his right to stop a slow roller, were out of the question. The same thing happened to Cisco in Senior League. I saw it when I was the JV coach and the varsity coach (my 'boss' who politely listened to my suggestions and then did what he wanted anyway) put a great lefty athlete in right field and a mediocre player at first because he could hit a little. I saw it when I was the varsity high school baseball coach, and other teams had the big guy on first.
Two MLB examples: In Chicago, Frank Thomas was expendable as a mediocre first baseman but prolonged his career as a DH. A more glaring example - David Ortiz was the focus of some anxious speculation among Boston fans last year when it was decided he would play first in the World Series. The Red Sox replaced a good-hitting, reliable first baseman (Kevin Youkilis) with a power-hitting DH who had not played first in a long, long time ... so long in fact that some journalists speculated that he would have to borrow a mitt!
And there you have my theories about Gil and Jackie, and the Modern First Base Coaching Theory Hypothesis. My theories may be all wet, but at least they were free!

Yes, I still have my first glove.