You need any backup to recover database in case of failure. So, if you believe nothing going to happen after you have full backup – feel free to overwrite it. However, as a DBA with a long history I would recommend to keep:

-Four full tested backups on tapes off-site;

-All differential backups since last or last plus one full backup (otherwise there is no way to do recovery to certain time from last full). All of them on hard drive for fast recovery and at least some off site;

-Last two days of transactional backups.

With that schedule you can recover any problem that happen up to two days ago, some problems that happen from last full backup, data in condition of up to a month ago. Also, you can create rotation of backup media according to schedule.

Two hours or five minutes depends on the business requirements: if your business cannot afford almost any losses (banking) you need not only every minute logs but also mirroring off site, if your business can recover from 24 hour data loss – you do not need logs on every two hours.

Same or separate folders: it is up to you. I prefer three different so it would be easy to find what I need and clean up old files.

What is the reason to do full backup on Monday night? Does it mean that database has a lot of critical updates on a weekend? If not, do full on Saturday or Sunday night.

Also, I would recommend reading books for the MCITP: Database administrator self-preparation. A lot of useful information and have answers to your questions.