'I have little money' and 'I have a little money' have slightly different meanings in English. Are they both valid translations of मेरे पास थोड़े पैसे हैं। ?
No. "I have little money" would be translated in a very different manner.
The sentence given in the question can be translated as - "I have some money", "I have a little money" etc
That seems to imply that you have quite a lot of money, through the art of understatement.
It is grammatically incorrect. ''Less" is the comparative degree of "little". So, if you use the word "less", it should be compared with something else. For example, "I have less money than she does."