The measure word 个 is often said to be the general-purpose measure word for nouns that the speaker doesn't know its measure-word. On the other hand, some nouns have well-established measure words.
Let's say, the word 房子 has the following measure words:间，座，栋
Yet, I've seen genuine Chinese texts that use 个 in front of 房子.
Is using 个 before a noun which has its special measure-word, grammatically correct? How would it sound?
I agree with the other two answers in general. But my opinions on the examples are slightly different. More specifically, I feel that
一个房子acceptable (depending on the context) but a little awkward; 一栋房子is probably the most natural, but 一座房子is probably also ok although I cannot think of a common situation for such use;
一个牛肉炒饭，一个鱼汤 acceptable, but not as common (ok to say 来个牛肉炒饭、来个鱼汤 ）, a better (generic) way would probably be 一份牛肉炒饭、一份鱼汤；
一个部队 seems not acceptable to me (maybe because 部队is a collective noun); 一支部队seems natural to me.
Please also note that the use of measure words is probably dialect- or even region-dependent: northern dialect vs. southern, HK people & taiwan may be different. For example, I've heard a taiwan friend of mine saying 一粒电池 (not sure if he is typical) while most mandarin speakers from the mainland would say 一节电池.
Also as pointed out in other answers, the "correct" use of measure word is probably not so important in most occasions, esp., for a foreigner/language learner, and I've seen the abuse of using 个everywhere even by native mandarin speakers.
I would say no, if there's a specific measure word for a noun, using 个 would be ungrammatical.
The measure words acceptable for 房子 are 间，座 and 栋， but not 个
The measure words for 铅笔 is 枝. 个 is unacceptable
However, some nouns can have specific measure words and use the general measure word as well.
Example:
A common measure word for 部队 is 支 as in 一支部队 (treat 部队 as a branch of an army), but 个 is also acceptable as in 一个部队 (treat 部队 as a unit)
In short, if an object can be counted as something that can use 个 as a measure word, it is o.k. to replace the common measure word with it
Example:
A waiter taking orders from a customer may say:
一碟牛肉炒饭，一碗鱼汤 = a plate of beef fried rice, a bowl of fish soup
一个牛肉炒饭，一个鱼汤 = a beef fried rice, a fish soup
牛肉炒饭 and 鱼汤 here refer to dishes (菜式 as in 一个菜式), thus 个 is acceptable as their measure words in this context.
Your questions and my short answers:
Is using 个 before a noun which has its special measure-word, grammatically correct?
No, it is not grammatically correct.
How would it sound?
It would sound ungrammatical, but most of the time, you would be understood.
Now, the longer answer:
I am assuming that you're asking from a learner's perspective. If you don't know what the measure word for a specific noun is, what do you do? Is it OK to substitute 个? Is it better to leave it out? Is it better to keep silent and wait till you find out the proper measure word?
I believe learning a language is a long process, during which mistakes are made and risks are taken. If you are in a very formal situation, and any kind of grammatical mistake will be highly embarrassing, then perhaps keep silent and and wait till you find out the proper measure word to use.
In everyday life, however, when you're among friends, or friendly strangers, then perhaps treat it as a learning opportunity, use 个 and if possible, ask if that is correct. Using something, even an incorrect measure word, is always better than leaving out the measure word altogether.
Learning the grammar is of course important. But language learning is not just about grammar. It's also about communicability. Communicate, learn, then communicate properly. It's a long and continuous process.
Having said that, many nouns do have semantic cues that can help you make an intelligent guess as to the correct measure word. Take your time, learn those, and build your confidence. Hope that helps.
The basic grammatical function of classifiers (or measure words) is to construct a quantifiable occurrence of the notion related to the noun. For discrete quantity, we use 个，条，支，块，粒... etc, for non discrete quantity, we use 些. For example:
In some contexts where quantity is not relevant, we do not use classifiers with the noun. For example:
In some other contexts where quantity is involved, we must use classifiers:
The generic classifier is 个, which functions plainly as a grammatical element. We usually advice new learners to use it as the default classifier: when you need to use a classifier and you are not sure which one to use, use 个.
As for the other classifiers like 条，支，块，粒... etc, on top of the grammatical function of constructing a discrete quantifiable occurrence, they have another lexical layer which is related to the properties of the object denoted by the noun (notably the shape or the form of the object). This lexical layer is a matter of vocabulary: to master the use of the various classifier, one has to build up his vocabulary. And as vocabulary varies from region to region, the set of classifiers you have learned may not be the same as the ones used by other Chinese speakers.
