No, they will all work together if you do it correctly (and other developers don't do anything silly)
For example, in MyAddOn_actionIndex, you should read input, filter, etc. whatever you want and then you set like
[php]$conditions['MyAddOn_type'] = "team";[/php]
And then in your MyAddOn_Model_Forum, you should override prepareXConditions too. In this example, you should do something like this
[php]
public function prepareForumConditions(array $conditions, array &$fetchOptions){
$result = parent::prepareForumConditions($conditions, $fetchOptions);
$sqlConditions = array($result);
if (!empty($conditions['MyAddOn_type'])) {
// our condition is set
$sqlConditions[] = 'forum.type = ' . $this->_getDb()->quote($conditions['MyAddOn_type']);
}
// do some other conditions if you want
if (count($sqlConditions) > 1) {
// got more than one, must have some custom conditions...
return $this->getConditionsForClause($sqlConditions);
} else {
// nothing unusual, just return the parent result
return $result;
}
}
[/php]
If all add-ons work this way, they will work together just fine 