Science

Follow the link below for colleges and check out details for admission:
https://www.shiksha.com/science/colleges/b-sc-agriculture-colleges-Coimbatore.

Well, its not possible to join DRDO after B.Sc. Geology for scientist position. DRDO used to conduct SET exam for entry of Scientist but this year they have resorted to GATE score. This may become rule now on wards. GATE score will be used to shortlist candidates for interview and then POINTS course I. e. Post Induction training school at DIAT Pune followed by posting to different laboratories spread across India. Since GATE has become a standard for many PSUs and now to DRDO, you may wish to have best GATE score possible to join DRDO.

The mode of selection can be +2 or an entrance exam depending on the course and college. Maximum states follow an 85% to 15% quota rule for the admission i.e. 85% for the state candidates and 15% for the rest of India.

I think DTU doesn't offer M.Sc programs:
Kindly visit this link:
http://dtu.ac.in/Platforms/academic_PG/choice.htm

Good colleges/universities for M.Sc. in microbiology are:
Fergusson College, Pune
Oxford College of Science
Ramnarain Ruia College, Mumbai
St. Francis Degree College For Women
Garden City College
Lady Brabourne College
Kristu Jayanti College, Bengaluru
Ganpat University, Mehsana
Marwadi University
SIES College of Arts, Science, and Commerce, Mumbai
CMS College of Science & Commerce, Coimbatore.

Yes, if you fulfil the eligibility criteria:
This course is a research based graduate degree. Candidates need to have passed graduation from a recognised university as aminimumeligibility criterion to be fulfilled for the programme. Admission to various nanotechnology colleges is either entrance exam based or merit-based.

Yes, sure.
Kindly visit the link mentioned below:
http://andhrauniversity.edu.in/coursesoff.html
You can also visit the website for further details.

College change or university change after first year is completely dependent on college and university norms, some university allow it and some won't, plus, you need to have good scores in first year to apply for the same. For further details, contact the college authorities.