[GENEAL] dynamically changing table

Hi,
In the next project I'm going to have a number of colums in my tables,
but I don't know how many, they change. They all use integers as
datatype though.. One day, I get 2 new columns, a week later I loose
one column, and so on in a random pattern.

I will most likely have a few million rows of data so I just wonder if
there are any problems with running
alter table x add column .....
or
alter table x drop column .....

Adding a column, will it place data "far away" on the  disc so that
select * from x where id=y will result in not quite optimal
performance since it has to fetch columns from a lot of different
places?
Will deleting a column result in a lot of empty space that will anoy
me later on?

Are there any other clever solutions of this problem?