Oh, many things happened while I was writing this... still I hope you'll find it useful, Fazor.
Creo que hoy es lunes. / Sé que hoy es lunes.
I wouldn't say they're interchangeable...
"Creo" still means you're not sure. Maybe you've been on holidays and today it's the same as the days before so you can't tell at first if it's Monday or not.
To say "sé que hoy es lunes" needs a much more special reason to be said. It would mean you don't have access to a calendar and you had to count the days on your own...
If you want to imply you're sure today it's Monday, you simply say "Hoy es lunes".
As for the Indians example, you're right.
Creo que
los
Indios de Cleveland
perdieron ayer.
Usually, the team names that sound similar in Spanish are translated (Cardinals-Cardenales, Indians-Indios, 49ers-"cuarentaynueves", etc.).
Not the same case of names like the Raiders, the Lakers, the Dodgers... which are kept in English because it's hard to associate translation and name.
...And the verb is conjugated in plural ("perdieron"), because you said "los Indios"; if you had said "el equipo", "perdió" would have been correct.
Sé que
los
Indios de Cleveland
perdieron ayer.
This sentence is correct, although since this happened yesterday, for reasons of style, I would suggest you rather said "Supe que los Indios de Cleveland perdieron ayer".
If the match were happening now, it would be fine to say "Sé que los Indios de Cleveland están perdiendo".
