1) It is not recommended to install multiple features without careful analysis of them. Actually products are carefully analysed and proven features that are grouped together to solve common problems. As an example if you combine AS + ESB features yes you can have it but it may result in poor performance in ESB since your services are hosted in the same server too. (Likewise there are many negatives). Further some of the combinations are not properly tested in an production environment and will have some unidentified issues, But released products are tested properly and used in production too.
2) Not compatible. You cannot install 3.x features in 4.x.
3) You may refer this for product overviews http://wso2.com/products/carbon/release-matrix/