OP, look at it this way, I feel that typically "适合" has a bit of a verb meaning, having a "适". I feel that this statement is the equivalent to a verb.
With regards to your first two statements, the first one works fine with "适合", but the second one, if you say "我在找一个合适我的女孩" (~I am looking for a girlfriend that I find suitable) is passable in colloquial speech, though it might stand out awkwardly.
The former is more like an adjective.Notice its "适" after "合",Chinese expression(“适合” and "合适"are two Chinese expression)
For us the second word is like a “助词”(auxiliary word and it's like a modal particle) at least for me,so the sencond word has no means.
It's a habit when you express emotion or something use a expression with at least two similar word together that the real meaning of this Chinese expression only with the first word（sometimes it maybe the others,it depends the situation and your comprehension）
For convenient,you only need to think the first word “合” or "适".
"合" in these expressions is a adjective, when you use it in expression most of the time it
is a adjective,so think what adjective used for will help you understand it.(It also has mean of verb but you need not to know this time)
And I feel that the 适 in the latter can sometimes have the meaning of a verb. (Looking for what they have discussed in the second hyperlink they also get confused about these word now and then)
This is a Baidu discussion of the two
This is another form discussion on the differences between the two from Baidu Post Bar
I hope this helps you. Chinese is a language you have to pay close attention to ponder its meaning.