适合 is broader. It can include a lot of different situations. 相称 is almost always focused on appearance or the way something or someone looks. 相称 is narrow. 这个外套和你的年龄不相称。For example, this sentence implies the jacket you're wearing doesn't "look" appropriate for your age.

相称有一点表示匹配，对称的意思，比如你的能力与职位相称。适合比较general。但是适合与相称放在句子里的位置不太一样，因为相有表示对比的意思。你可以说你的能力与职位相称，但不能说你的能力与职位适合，应改成你的能力适合你的职位。it seems i made the thing too complicated, hope the example will help you.

这两个词，大部分情况下是可以通用的，但"相称"带有"地位或能力"比较的意思，例句：1、这辆破车和国王的地位并不相称。2、这辆破车并不适合给国王用。 3、他这么老了，不适合做剧烈运动。

相称 has a connotation that someone or something need to "match" their class, income, fame, appearance or environment etc. It's usually used in a grammatical structure: 和 ... 相称 (literal translation: to match ...) or 与 ... 相称 (a bit more formal than the previous one). 适合 sounds more like suitable, to be fit for someone or something.