Gst liability

Firstly, The any person who's turnover less than 20lakh is not liable to registered under GST. However Such person can take Voluantry registration Under Registration.
In your case it is not clear that such person is registered under GST or a not , if such person registered under gst He is liable to pay gst on Sale made by him
& your 2nd query solution that Every registered person Whose turnover during the financial year less than 1.5crore can opt for composition Scheme

1- you can take voluntary registration in gst . no mendetory to take reg under 20 lakh ..
if you reg in gst then you have to pay gst liability

2- yes you can take compostion reg . turnover 1 crore limit ... composition not for service provider expect restru and hotals