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.
楼主,是这样的,我感觉一般"适合"有点动词意味,有一个"适"字,这个词我觉得相当于是个动词,就你那两句而言,第一句用适合也是可以的,但是第二句,就感觉而言如果你说“我在找一个合适我的女孩”这样口语也说的过去,就是显的有些别扭。前者更像一个形容词,后者的"适"我感觉有时候做动词意义。
中文有的时候有两个意义相近的词表示一个意思,这个时候第二词更像是一个虚的(没有实际意义),注意其中一个词的意思即可,一般来说第一个为真正本意的概率大一些(当然,其他情况也是有的)
所以这里你注意两个词组的第一个词即可.
这是百度关于二者区别的一个连接
这是某贴吧的讨论
希望对你有帮助,中文更讲究一个语境和形义思维。