Yes, it should look as if it is part of a set, i.e., all the pieces should reflect the save overarching corporate identity.

The short answer is keep it as a suite, bearing in mind that a letterhead obviously has a different purpose to a comp. slip and a business card.

The relationship of letterhead to comp. slip is much closer than the business card in that they will need the same information, postal address, registered address and registered number, probably with a generic telephone, fax, e-mail and url, wheras a business card has some of this information but with the name of the individual, with a direct line and e-mail and is not primarily intended to be written on.

The logo should be the same unless you have a clear reason why it shouldn’t be, although you might consider if you have a particularly vertical logo you might require a horizontal logo for some applications, for example van graphics or website masthead. The treatment of the text needn’t be exactly the same although I would keep to the typeface family, the proportions of point size and leading the same. It really depends on the design and requirements of the client, as a student (and I am guessing this a college project) you are probably best off starting with keeping the elements the same.

Try searching for stationery projects in some design agency portfolios.

Tim

so does it mean that if a business card is portrait, then a compliment slip should be too? is there anything such as a vertical comp slip?

There are vertical comp slips, the usual format is landscape.
The problem with portrait business cards is that one will occasionally come across somebody with a double barrelled name whether inherited or due to a marriage and that can present a problem with the name, but more of a problem is the e-mail address a long name joined to a long e-mail url can be too long for portrait to contain in one line.
Tim