I dont think i have seen anyone post this yet, but the 1b in the ratings refers to all hits, not just singles. The 1b portion of the part effects is used at a different portion of the event tree than the rest of the numbers.
for example (all imply large sample sizes)- comparing a (0,0,0,0,0) field and a (2,0,0,0,0) field - there would be more hits at the +2 1b park, but the % of hits that would be 2b, 3b and HR would be similar. there would be more 2b, 3b and HRs in the park with +2 1b simply because there are more hits.
If you were in a (0,4,0,0,0) park, you would have roughly the same number of overall hits, but the % of those hits that were doubles would be much higher in the +4 2b park (likely at the expense of singles).
If you move to Alb (4,4,4,1,0), you are going to have many more hits in general, but then the % of those hits that are 2bs and triples will also go up, basically creating a multiplying effect. You will end up with more HRs in Alb as well, because even though the (1,0) doesnt add a ton, the number of hits is higher and the % is the same.
For anyone who has played SLB, this is fairly well known.
When constructing teams in + 1b parks, I tend to overemphasize contact and de-emphasize batting eye (its still one of the most important, just it becomes less in a hitters park) because putting the ball in play more often takes better advantage of the park effects. In pitchers parks, extreme power, batting eye and speed/BR become the primary drivers. Extreme power (95+) does not appear to be muted much by -HR effects and batting eye/speed/br work as previously described...turning singles and walks in to doubles and triples.