Great questions!

[quote=Ajmal Mohamed;1148]
1. Is it possible to add custom fields to any entities on Hotcakes like products, categories etc? Not product variants but custom fields. We want to add values to those custom fields from Store Admin and need to display the same on front end.
[/quote]

You can do this with product type properties:
https://hotcakescommerce....duct-Type-Properties
https://hotcakescommerce....erties-for-a-Product

[quote=Ajmal Mohamed;1148]
2. Is it possible to assign user specific pricing to a product? So, when those users logged in and browse those products then that price will displayed. Normal price will displayed for other users.
[/quote]

You can do this with price groups and/or promotions:
https://hotcakescommerce....8692006-Price-Groups
https://hotcakescommerce....210654943-Promotions
https://hotcakescommerce....tions-Qualifications

[quote=Ajmal Mohamed;1148]
3. Many links from store dnnsoftware and hotcakes portal are pointing to paid cloud version. Are there any extra features on Cloud version other than hosting on cloud? If yes, is source code available with paid version and can we integrate that into our project rather than hosting separately on cloud? Where we can find the whole difference (comparison) between free open source version and cloud version?
[/quote]

The cloud version is not different software. The source code is the same. The benefits you get from the cloud version include year-round support and Hotcakes/DNN upgrades. If you want to host it yourself, you're able to do that as well.